In wireless communication systems, antenna arrays are used at devices on one or both ends of a communication link to suppress multipath fading and interference and to increase system capacity by supporting multiple co-channel users and/or higher data rate transmissions.
For example, in time-division duplex (TDD) systems, a base station (BS) device and each of a plurality of mobile station (MS) devices are equipped with a plurality of antenna. In general, the BS device may be referred to as a first wireless communication device and a MS device may be referred to as a second wireless communication device, or vice versa.
For cost and other reasons, one device on the communication link, e.g., a MS device, may use all of its plurality of antennas only for receiving signals. For example, an MS device may have N plurality of antennas and N receivers, each of which is configured to receive a signal from a respective one of the N plurality of antennas. However, in order to reduce the cost, the MS device may have less than N transmitters since each transmitter requires its own power amplifier and related circuitry. Thus, the MS device cannot transmit using all of its plurality of antennas. In one particular example in which an MS device has four (N=4) antennas, it has four receivers but only two transmitters. Thus, in this example, it can be said that the MS device has two active antennas and two inactive antennas for purposes of transmissions. Consequently, when a BS device receives signals from that MS device, it cannot directly estimate the channel with respect to all of the antennas of the MS device because the transmissions from the MS device are made from less than all of the antennas of the MS device.
A need exists to derive knowledge of a communication channel between a first device having a plurality of antennas and a second device having a plurality of antennas when the second device transmits to the first device using less than all of its antennas. Using this knowledge of the channel, more accurate values for beamforming weights can be computed for transmitting signals from the plurality of antennas of the first device to all of the plurality of antennas of the second device, such as in a multiple-input multiple-output (MIMO) communication system.